Make the egg something special. A gemstone the size of a basketball, perhaps. Is this a little D dragon, or a Dragon? I don't think the big Ds are reproducing much anymore, but little Ds are probably making a comeback with the end of knights hunting down dragons to save damsels (or, as was more likely, the dragons are coming back from various human pogroms caused by the theft of livestock).
Dragons laying eggs is fine by me.
I don't know for sure if Dragons can interbreed, but I think they can. My theory about Drakul (and by some extension his son Dracula) is that he was sired by a Big D Dragon. As far as the little D fire-breathing lizards, I don't know if they could or not.